The remote sensing of atmospheric trace gases is of increasing importance. In particular, obtaining accurate measurements of atmospheric trace gas species, such as CO and CO2, from an aircraft or spacecraft platform is essential for improving the scientific understanding of global atmospheric chemistry, climate impacts, and the atmospheric component of the global carbon budget.
In order to perform such remote sensing, Fabry-Perot interferometers and other types of high spectral resolution instruments such as Fourier transform spectrometers and grating spectrometers have been identified. In addition, airborne Fabry-Perot instruments have been tested, and measurements from space-based platforms have been proposed. However, the field of view over which a Fabry-Perot interferometer can provide accurate measurements has been limited. Because of this limited field of view, such devices have generally required a scanning mechanism to provide appreciable field coverage. The introduction of a scanning mechanism reduces scan efficiency, which leads to reduced signal integration time, therefore requiring relatively large aperture sizes in order to provide a given signal to noise ratio. This limits the number of spectral samples taken of any one portion of the atmosphere within the field of view of the device and has hindered the wide acceptance of Fabry-Perot interferometers for space-based remote sensing missions.
These limitations of Fabry-Perot interferometers and other optical cavity interference filters are fundamental. The transmission function of the Fabry-Perot interferometer, and derivative optical cavity filters, is a function of the optical path through the etalon cavity. In particular, the wavelengths at which passbands of the Fabry-Perot etalon cavity are formed are highly dependent upon the angle at which collected light has passed through the filter. Specifically, as the angle of incidence of light with respect to the filter changes, the path length of that light through the filter cavity or cavities also changes. The result is a shift in the transmitted wavelengths. Because of this shift, it has been necessary to collect light provided to the filter from over a relatively narrow field of view (e.g., less than 0.2 degrees) in order to prevent the passbands of the filter from moving off of wavelengths corresponding to the spectral lines of absorption of a gas being measured.
In order to perform remote sensing over a wide area, a sensor incorporating a spectrometer device such as the Fabry-Perot interferometer or a thin-film filter can be deployed as part of a moving platform. The forward motion of the platform provides the spatial coverage of the scene, while the spectrometer defines the spectral coverage. Alternatively or in addition, the device can be mechanically scanned. As yet another approach, the index of refraction and/or the spacing between the opposing mirrors forming a Fabry-Perot interferometer can be adjusted or the angle of the etalon filter can be adjusted with respect to the incoming light to provide wavelength scanning. However, each of these approaches requires the instrument to dwell over a fixed location on the ground for a period to provide the wavelength scanning of a fixed scene. These approaches are not capable of providing increased signal integration times nor increased signal to noise ratios when continuous ground coverage is desired. Furthermore, approaches that rely on mechanical adjustments to the etalon are unreliable and are difficult to implement.